This invention relates to bobbins, transformers, magnetic components, and methods.
FIGS. 1A and 1B show, respectively, a top and side view of a transformer 10 of the kind described in U.S. Pat. No. 5,719,544 (xe2x80x9cTransformer With Controlled Interwinding Coupling and Controlled Leakage Inductances and Circuit Using Such Transformer,xe2x80x9d Vinciarelli et al., assigned to the same assignee as this application and incorporated herein by reference, the xe2x80x9ctransformer patentxe2x80x9d). The transformer comprises two bobbin assemblies 1A, 1B, each comprising an electrically conductive winding 2A, 2B wound over a non-conductive bobbin 4A, 4B. The two windings are linked by a magnetic medium comprising two core assemblies 11. Each core assembly comprises an electrically conductive medium 12 selectively arranged over the surface of a permeable core piece 6 (e.g., by means of platingxe2x80x94see, for example, U.S. patent application Ser. No. 08/941,219 filed on Oct. 1, 1997xe2x80x94or use of formed sheets or foils). The faces 8 of the core pieces 6 are free of conductive medium and a slit is provided along the inner periphery of the core assemblies (not shown), thereby preventing formation of a xe2x80x9cshorted turn.xe2x80x9d The conductive medium 12 constrains the transformer leakage flux to lie within the region confined by the conductive medium. As discussed in the transformer patent, such a transformer has a number of benefits: it exhibits much lower leakage inductance than similar transformers without a conductive medium; the widely separated windings exhibit low interwinding capacitances; the placement of the windings provides for easy removal of heat; and many different transformers, varying in terms of turns ratio and leakage inductance, may be constructed from relatively few common parts (e.g., bobbins, core pieces). The magnetizing inductance of the transformer may be set by means of a gap 16 in the magnetic path (a portion of the bobbin 4B and winding 2B are shown cut away to show the gap).
In other transformer embodiments, described in the transformer patent and shown in FIG. 2, extensions 20 of the permeable magnetic material may be used to provide a low reluctance path for leakage flux 21 in the region between the core halves, thereby providing a greater possible range of leakage inductance. Such extensions 20 may also be covered with a conductive medium.
As shown in FIG. 3, a saturable inductor 22 is sometimes placed in series with a winding 26 of a transformer 24 in a switching power supply. In some applications, the saturable inductor is used to limit rectifier 32, 33 reverse recovery currents and attendant conducted and radiated noise. Such an inductor may also be used in a converter comprising an xe2x80x9cactive clampxe2x80x9d core resetting circuit 30 (of the kind described in U.S. Pat. No. 4,441,146, xe2x80x3Optimal Resetting of the Transformer""s Core in Single-Ended Forward Converter, Vinciarelli, assigned to the same assignee as this application, incorporated by reference) to provide a high impedance load on the transformer winding for a short time following turn-on of the main switch 28, thereby allowing the xe2x80x9cmirroredxe2x80x9d flow of transformer magnetizing current to more fully charge and discharge parasitic capacitances than would otherwise be possible without it and allow for zero-voltage switching operation. The number of turns on the saturable inductor 22 will depend on the required xe2x80x9cvolt-secondxe2x80x9d rating and will, for a given transformer configuration, vary as a function of the output voltage of the converter. To maintain a fixed xe2x80x9ctime to saturationxe2x80x9d, the number of turns on a saturable inductor will, for a given saturable core, need to increase in proportion to transformer output voltage. Thus, different saturable inductors are generally required for different output voltage settings.
In general, in one aspect, the invention features a bobbin adapted to support a winding on a permeable core and having a wall that provides a confined thermally conductive channel that causes conduction of heat along a predetermined path from the core to a location outside the winding.
Implementations of the invention may include one or more of the following features. The bobbin may have an electrically insulating wall surrounding a hollow interior space, the electrically insulating wall including segments having different thermal conductivities to provide the confined thermally conductive channel. The confined thermally conductive channel may be provided by ceramic (e.g., alumina). One of the segments may be plastic. A solderable metal coating of the bobbin may provide the confined thermally conductive channel and may be attached to the permeable core. The confined thermally conductive channel may have a thermal conductivity greater than 1 BTU/(hourxfootxdeg.F) while another segment of the bobbin may have a thermal conductivity less than 1 BTU/(hourxfootxdeg.F)).
In general, in another aspect, the invention features a magnetic component that includes the bobbin and a permeable core.
Implementations of the invention may include one or more of the following features. The permeable core may include separable core pieces that define a magnetic path. The ends of core pieces may be separated by a gap. The core pieces may include a conductive medium on portions of their surfaces. One of the segments may be attached (e.g., by epoxy or solder) to the core pieces and may set the gap. The conductive medium may be copper.
In general, in another aspect, the invention features a leakage inductance transformer that includes the bobbin, a winding surrounding the bobbin, and a permeable magnetic core having a magnetically permeable segment which passes within the bobbin to form a flux path that couples the winding.
Implementations of the invention may include one or more of the following features. A magnetically permeable leakage lug and a permeable magnetic slug may be located outside of a hollow interior space enclosed by the bobbin. The slug may lie in a flux path defined by, and be permeably linked to, the leakage lug. The slug may be a saturable magnetic material.
In general, in another aspect, the invention features a method of setting a value of magnetizing inductance in a transformer comprising a winding bobbin having segments one of which is more thermally conductive and core pieces having ends that are separated by a gap. The method includes adjusting the gap until the value of magnetizing inductance has been set, and attaching a segment of bobbin to the core pieces to maintain the gap.
Implementations of the invention may include one or more of the following features. The adjusting may include moving one core piece to adjust the gap while measuring the magnetizing inductance and stopping the movement when the measurement of magnetizing inductance is essentially equal to the pre-determined value. The attaching may include providing a bonding medium in the region between the surfaces of the core pieces and processing the bonding medium to cause it to set. The bonding medium may include thermally setting epoxy, or solder. The processing may include heating the bonding medium by passing a magnetic field through the gap.
In general, in another aspect, the invention features a transformer structure that includes a bobbin that defines a hollow interior space and has an outer surface configured to carry a winding, a permeable core that lies within the interior space in a position to couple the winding, a permeable slug that provides a permeable path outside of the hollow interior space and does not couple the winding, and an electrically insulating coupler interposed between the slug and the winding to electrically insulate the winding.
Other advantages and features will become apparent from the following description and from the claims.